The present invention relates generally to thermal sensing circuits with voltage reference circuits, and more specifically thermal sensing circuits implementing bandgap voltage reference circuits.
Thermal sensing circuits are sometimes utilized to monitor substrate temperature in electronic systems. For example, a thermal sensing circuit can be used to monitor a substrate temperature of a chip or processor. When the substrate temperature exceeds a predetermined temperature threshold, the thermal sensing circuit might, for example, signal circuitry of a computer system so that corrective action, such as throttling back or shutting down the processor, may be taken to reduce the temperature. Otherwise, the processor could overheat and cause the processor to fail.
Thermal sensing circuits are typically fabricated on a separate discrete integrated circuit, or chip, and are coupled to one or more external pins of the processor. Using these external pins, the thermal sensing circuit can bias a thermal sensing element, such as a diode, of the processor into forward conduction and sense an analog voltage across the thermal sensing element. The thermal sensing circuit may convert the analog voltage into a digital value that reflects the substrate temperature. The thermal sensing circuit can then determine when the substrate temperature surpasses a specified temperature threshold.
FIG. 1 is a block diagram of a conventional thermal sensing circuit that includes a trimming circuit 5, a reference voltage generator 10 that generates a reference voltage which corresponds to a fixed thermal threshold, a thermal sensing element 30 that generates a base-to-emitter voltage that is proportional to temperature, a comparator 40 that compares the reference voltage to an output voltage of the thermal sensing element, and a control circuit 50 that generates an indicator signal when the temperature that is sensed exceeds a thermal threshold T1.
FIG. 2A is a graph of bandgap reference voltage and base-to-emitter voltage as a function of temperature. As shown in FIG. 2A, the thermal threshold T1 is determined by the intersection of the bandgap reference voltage and the base-to-emitter voltage Vbe. Accordingly, the temperature threshold T1 can be increased by lowering the reference voltage or can be decreased by increasing the reference voltage.
FIG. 2B is a timing diagram that shows the relationship between timing of an indicator signal generated by the thermal sensing circuit of FIG. 1 and temperature. As shown in FIG. 2B, the temperature threshold T1 is significant, since the intersection of the temperature threshold line with the measured temperature plot (shown as a triangle shaped signal) determines the points at which the indicator signal OUTPUT_SIGNAL will transition from a low level to a high level and from a high level to a low level. The indicator signal OUTPUT_SIGNAL transitions from a low level to a high level when the measured temperature plot (shown as a triangle shaped signal) has a positive slope (i.e., increasing temperature) above temperature threshold T1 and transitions from a high level to a low level when the measured temperature plot has a negative slope (i.e., decreasing temperature) below temperature threshold T2.
Bandgap voltage reference circuits are sometimes utilized to provide stable reference voltages that do not vary despite temperature variations. Bandgap voltage reference circuits utilize the characteristics of the bandgap energy of a semiconductor material to provide a stable reference voltage. The bandgap energy of a semiconductor material is typically a physical constant at zero degrees Kelvin. However, as the temperature of the semiconductor material rises from zero degrees Kelvin, the bandgap energy of the material decreases, and a negative temperature coefficient is displayed.
The voltage across a forward biased PN junction generally provides an accurate indication of the bandgap energy of a material. As the temperature of the semiconductor material increases, the voltage across a forward biased PN junction will decrease at a rate which depends upon the cross-sectional area of the particular PN junction and the specific semiconductor material being used.
Two forward biased PN junctions that are made of the same semiconductor material, but that have different cross-sectional areas, will have voltages that vary at different rates when the temperature of their respective PN junctions change. Nevertheless, these voltages can be traced back to the same bandgap voltage constant at absolute zero.
Conventionally constructed bandgap voltage reference circuits can utilize the voltage relationships (between these two forward biased PN junctions) to achieve a relatively temperature insensitive output voltage. Examples of such circuits are shown in FIGS. 3 and 5A-5C, which are discussed in greater detail below. Such bandgap voltage reference circuits utilize a feedback loop in conjunction with an operational amplifier, that is utilized as a differential amplifier, to generate a reference voltage. The feedback loop maintains two input nodes of the differential amplifier at approximately the same potential at steady-state. The non-inverting input of the differential amplifier can be coupled to a reference potential through a first PN junction, such as a diode or transistor. The inverting input of the differential amplifier can then be coupled to the reference potential through a resistor and a second PN junction that has a larger cross-sectional area than the first PN junction. The second PN junction can be constructed using a plurality of the first PN junctions, such as an array of diodes connected in parallel.
During circuit operation, substantially equal currents are forced through the first and second PN junctions. By selecting appropriate component values, a bandgap voltage reference circuit can be provided that balances the negative temperature coefficient associated with the first PN junction with a positive temperature coefficient associated with the difference in the PN junctions to thereby generate a relatively temperature insensitive output voltage.
FIG. 3 illustrates a conventional bandgap reference generator circuit 10. The bandgap reference generator circuit 10 includes an amplifier 11, a positive voltage supply rail 8, a negative voltage supply rail 9, a current source transistor 12, a resistor 13, a diode 14, a resistor 15, a resistor 16, and a diode array 17A-17N. The amplifier has two input signals, voltage Va and voltage Vb, which are fed back from nodes 2 and 3, respectively, to form a control loop. The output of the amplifier 11 is connected to and drives the gate of transistor 12 with a bias voltage which causes a current to flow through resistors 13, 15, 16 to generate voltages Va, V6, Vref, respectively.
The source/drain of transistor 12 is coupled to a positive voltage supply rail 8, and the drain/source of transistor 12 is coupled between resistor 13 and resistor 15. Resistor 13 is coupled to the anode of diode 14 and the cathode of diode 14 is connected to negative voltage supply rail 9. Voltage Va is generated at node N2 between resistor 13 and diode 14. Resistor 15 is connected in series to resistor 16 to form a voltage divider, which is connected to diode array 17A-17N. Voltage Vb is generated at node N3 between resistor R2 and resistor R3. The output of resistor 16 is coupled to the anode of diode array 17A-17N. The cathodes of each diode in the array 17A-17N is connected to negative voltage supply rail 9. The reference voltage Vref at node N1 is approximately 1.25 volts.
FIG. 4 is an electrical schematic of a conventional thermal sensing element circuit. As shown in FIG. 4, the thermal sensing element 30 comprises a constant current source 32 that is coupled to a diode 34 which has a negative temperature coefficient. The base-to-emitter voltage Vbe is measured at the node between the constant current source 32 and the anode of diode 34. The cathode of diode 34 is coupled to the negative voltage supply rail 9.
In designing such circuits, the stability of the reference voltage over voltage, process and temperature variation, among other factors, are very important to consider with respect to the temperature threshold. Generally, thermal sensing circuits are so affected by process variations that the calibration is required via fuse trimming/programming circuitry 5.
Integrating both the bandgap reference circuit 10 and the diode 34 is often very difficult since the 1.25 volt voltage of the bandgap reference circuit 10 is too high in comparison with the base-to-emitter voltage Vbe of diode 34. Moreover, the reference voltage generated by conventional bandgap reference circuits 10 tends to be fixed at a value of approximately 1.25 volts, which essentially eliminates any flexibility of the thermal threshold T1.
FIG. 5A is an electrical schematic of another conventional bandgap reference voltage generator circuit in which the value of the reference voltage can be set to either 1.25 volts or 1.25 volts*ratio of resistor 19 to resistor 13A. As shown in FIG. 5A, the bandgap reference generator circuit 10 includes an amplifier 11, an NPN transistor 12A, 12B, 12C, resistors 13A, 16, 18, 19, a diode 14 and a diode array 17A-17N. Amplifier 11 is responsive to inputs Voltage A and Voltage B. The output of amplifier 11 biases transistors 12A, 12B, 12C since the gates of transistors 12A, 12B, 12C are connected. The source/drain of transistors 12A, 12B and 12C are all coupled to positive voltage supply rail 8. The drain/source of transistor 12A is coupled to node N1 which is connected to a parallel combination circuit that includes resistor 13A and diode 14. Voltage Va is generated at node N1. The diode 14 is connected between the node and the negative voltage supply rail 9.
The drain/source of transistor 12B is connected to node N2 which is connected to a parallel combination circuit that includes diode array 17A-17N, resistor 16, and resistor 18. Resistor 16 is connected between node N2 and the anodes of each diode 17A-17N. The cathodes of diodes 17A-17N are connected to the negative voltage supply rail 9. Resistor 18 is connected between node N2 and ground. Voltage Vb is generated at node N2 and feedback to the amplifier 11.
The reference voltage Vref is measured at node N3 connecting the drain/source of transistor 12C to resistor 19, which is connected to the negative voltage supply rail 19. The bandgap reference circuit shown in FIG. 5A allows the reference voltage Vref to be changed between 1.25 volts and another discrete voltage that is the product of 1.25 volts and the ratio of resistor 19 and resistor 18. This allows the reference voltage Vref to have two distinct values.
FIG. 5B is an electrical schematic of another conventional bandgap reference voltage generator circuit in which the reference voltage can be set to either 1.25 volts or the product of 1.25 volts and the ratio of resistor 19 to resistor 20. This bandgap reference circuit includes the first amplifier 11A, second amplifier 11B, transistors 12A, 12B, 12C, 12D and 12E, a positive voltage supply rail 8, a negative voltage supply rail 9, a diode 14, a diode array 17A-17N, resistors 16, 19, and output resistor 20. The gate of transistor 12A is coupled to the gate of transistor 12B which is coupled to the gate of transistor 12C. The gate of transistor 12D is coupled to the gate of transistor 12E. In this embodiment, the first amplifier 11A has inputs Va and Vb, and the output of amplifier 11A drives the gates of transistors 12A, 12B, 12C. Similarly, the second amplifier 11B has inputs of Va and Vc and generates an output that drives the gates of transistors 12E, D. The source/drains of transistors 12A, 12B, 12C, 12D, 12E are coupled to positive voltage supply rail 8. Diode 14 has an anode that is directly coupled between the drain/source of transistor 12A and the negative voltage supply rail 9. Voltage Va is generated at node N1 connecting transistor 12A to the anode of diode 14. Resistor 16 is connected between the drain/source of transistor 12B and the anodes of each diode in the array 17A-17N. The cathodes of each diode in the array 17A-17N are grounded. Voltage Vb is generated at node N2 connecting resistor 16 to transistor 12B. Resistor 19 is coupled between the drain/source of transistor 12C and the negative voltage supply rail 9. The connection between resistor 19 and transistor 12C defines node N3. Node N3 is also coupled to the drain/source of transistor 12D, and the reference voltage is measured at node N3.
The drain/source of transistor 12E is coupled to resistor 20 which is connected to the negative voltage supply rail 9. Node N4 is disposed between transistor 12E and resistor 20, and generates the voltage Vc which is fed back to amplifier 11B. Va and Vc are the inputs of the control loop that includes amplifier 11B.
FIG. 5C is an electrical schematic of another conventional bandgap reference voltage generator circuit from U.S. Pat. No. 6,501,256 B1 to Jaussi et al. which shows a bandgap voltage reference circuit 1200 that simultaneously generates two reference voltages. VREF is generated relative to the negative voltage supply because current I3 passes through resistor 170 which is connected to the negative voltage supply. The bias voltage on node 132 produced by differential amplifier 130 is used to bias current source transistor 1210, which in turn produces current 1212 (I4). I4 is mirrored through the action of transistors 1214 and 1216 to produce current 1222 (I5). Current I5 passes through resistor 1218 to produce VREF2 relative to the positive voltage rail.
Accordingly, there is a need for thermal sensing methods and apparatus that implement bandgap reference voltage generator that can operate at a fixed operating point and that do not require elaborate fuse trimming or programming to calibrate the bandgap voltage reference generator. There is also a need for methods and apparatuses that can provide multiple reference voltages without unnecessarily consuming valuable chip layout space. It would also be desirable to thermal sensing circuitry that can eliminate the need for a separate thermal sensing element.